Combination devices, by means of which both magnetic resonance imaging and the simultaneous use of a second diagnostic and/or therapeutic modality are possible, are desirable in many application areas. Combinations of magnetic resonance scanners with X-ray equipment, nuclear medicine equipment (PET, SPECT) or radiotherapy equipment (for irradiating a tumor, for example) shall be cited only by way of example. If a second modality of said kind is to be combined with a magnetic resonance scanner, a great problem is the structure of the magnetic resonance scanner per se.
A simplification in terms of the structural embodiment of combination devices came with the introduction of what are referred to as open magnetic resonance scanners, in which there is a greater accessibility to the patient. In particular an embodiment in which two essentially cylindrical and/or rectangular coaxial magnet sections are used, the so-called double-doughnut configuration, is particularly suitable for allowing appropriate access to a second modality. For example, in a double-doughnut configuration, if the patient is positioned along the z-axis through the openings of the two “doughnuts”, access is possible in the axial plane between the two “doughnuts” or magnet sections.
One of the biggest challenges in this case is that the normally used cylindrical whole-body coils comprise a radio-frequency shield which essentially prevents, or at least severely restricts, the integration of a second modality, since the described access is blocked. Such whole-body coil arrangements are needed, however, on account of their outstanding properties, in particular with regard to susceptibility to failure.
Essentially three variants have been proposed in the past in order to solve the problem of unrestricted access to the patient: Firstly, the energy source and/or the detectors of the second modality can be disposed inside the whole-body coil arrangement. This, however, results in the considerable drawback that a very narrow structure exists and the introduced components of the second modality often affect the radio-frequency performance of the whole-body coil (and also of the gradients) and vice versa. In a second possible alternative it has been proposed to introduce the energy of the energy source through the two bores of the cylindrical or essentially cylindrical tunnel of the whole-body coil arrangement. This is not possible in all cases, since leads for the corresponding type of energy are not available or not compatible with the magnet resonance environment. Finally, it has also been proposed no longer to use a whole-body coil at all, though this results in a heavy loss of the field homogeneity and the robustness of the overall system.
In U.S. Pat. No. 6,466,018 B1 it has been proposed to use a birdcage coil without a radio-frequency shield. To ensure the coil conductors likewise do not create a disruptive influence, in that case with regard to the access to the patient, it has been proposed to embody the body coil as rotating, such that basically it can be rotated so that access to the patient is made possible. Due the absence of the radio-frequency shield, however, the magnetic resonance measurement is severely disrupted, and furthermore a costly and complex rotator device must be provided.